Fix race in runc exec

There is a race in runc exec when the init process stops just before
the check for the container status. It is then wrongly assumed that
we are trying to start an init process instead of an exec process.

This commit add an Init field to libcontainer Process to distinguish
between init and exec processes to prevent this race.

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

libcontainer/integration/checkpoint_test.go

@@ -110,6 +110,7 @@ func testCheckpoint(t *testing.T, userns bool) {
 		Env:    standardEnvironment,
 		Stdin:  stdinR,
 		Stdout: &stdout,
+		Init:   true,
 	}
 
 	err = container.Run(&pconfig)
@@ -205,6 +206,7 @@ func testCheckpoint(t *testing.T, userns bool) {
 		Cwd:    "/",
 		Stdin:  restoreStdinR,
 		Stdout: &stdout,
+		Init:   true,
 	}
 
 	err = container.Restore(restoreProcessConfig, checkpointOpts)